Radiology is a medical specialty that uses imaging to diagnose and treat diseases within a patient. A commonly used imaging technique is magnetic resonance imaging (MRI). MRI scanners use strong magnetic fields, radio waves, and field gradients to form images of a patient. One type of MRI is inversion recovery imaging which imparts T1-contrast in the acquired image by playing a spatially non-selective or selective inversion pulse. The time delay from the inversion pulse to acquiring (also called “reading out”) data, specifically to the acquisition of the image contrast-relevant line of the raw data space is known as inversion time (TI). During TI the magnetization recovers from its inverted state. Depending on the longitudinal recovery time (T1) of a specific tissue type or other matter, the respective magnetization has experienced a different amount of recovery at the end of the TI period. To obtain the desired image contrast between matter or tissue of different T1 it is crucial to correctly set TI. For example, in late gadolinium enhancement (LGE) TI should be set so that viable normal myocardium has very little signal at the time of acquisition so that it appears black to dark gray in the resulting T1-weighted image. This principle is also known as “nulling” of normal myocardium. In newer MRI applications, the IR pulse is executed together with other preparations such as magnetization transfer preparation or T2-preparation. For these applications, TI should be set so that the magnetization of more than one T1 species (for example blood, normal myocardium, and infarcted myocardium) are ordered in a desired manner. This is more challenging than nulling a single T1 species.
Currently, finding TI manually is a time-consuming, iterative process that requires a well-trained scanner operator. Even experienced operators frequently use suboptimal TI times, especially when a T1-shortening contrast agent has been injected in the patient's blood pool. The contrast agent is continually being filtered out of the blood pool by the kidneys (called renal clearance), but the removal rate is patient- and contrast agent-dependent. As a result, the T1 in blood and tissue changes after the injection of a contrast agent in a continuous but not completely predictable manner. Therefore, the TI needs to be constantly re-adjusted to obtain optimal image contrast. However, scanner operators often do not readjust TI due to a lack of time. For delayed enhancement imaging, where differences in contrast delineate regions of myocardial damage, this can lead incorrect depiction of damaged tissue and in the worst case to missed abnormalities and a wrong diagnosis.
With the introduction of the delayed enhancement sequence for imaging myocardial viability and infarction, the need arose to correctly set TI. The sequence uses an IR pulse followed by a time delay and a data-readout, and requires the injection of a T1-shortening contrast agent. The inverted magnetization recovers exponentially with T1, which is a tissue property. After the injection of the contrast agent, T1 in infarcted myocardium (irreversibly damaged tissue) is shorter than in viable myocardium. Therefore, these tissue types recover at different rates resulting in different signal intensities after the inversion and allowing their differentiation on T1-weighted images. The time delay between IR pulse and readout has to be set so that viable myocardium appears black to dark-gray in the image, indicating that it has no or little signal, also known as “nulled” signal. Infarcted myocardium appears bright due to its shorter T1. For sake of simplicity, this time delay can be regarded as the TI parameter. After the contrast agent injection, initial uptake and later washout lead to a continuously changing contrast agent concentration in blood and tissue, and concurrent T1 changes. That is why the inversion time may need to be continuously re-adjusted for achieving consistent image contrast across all times post injection. Some semi- or fully-automated methods exist for finding and setting TI appropriately (for nulling myocardium).
For at least the aforementioned reasons, there is a need for improved systems and techniques for determining TI and for adjusting TI during imaging of a subject.